"Just for fun"
... I guess that depends on what you count as being "fun".
I'm sure it's an incredible achievement - but hell, its "just for fun" for the creator - the idea of dicking about compiling this and getting it running, to end up with an OS about 90% less capable than what I currently use, is not something I would call "fun".
I mean, I get it - working with code can be cool - just the achievement in terms of the hours of sweat and horror spent, when you finally make a breakthrough, is often worth it - because developers are masochists.
For the developer, the learning process has almost certainly increased their skill set dramatically - for most of the rest of us, "That's nice, well done. Moving on now...."
Unless, of course, you are into making your own OS - in which case, if you are in that niche market, this is probably the most exciting project ever.
I do wonder, however, whether that time would've been better spent, improving the offerings we already have.
But hell, people spend their time in all sorts of incredibly weird ways, so who am I to judge?
/me heads off to sort collection of gravel by size, date and appealing textures...